1. Field of the Invention
This invention is concerned with the manufacture of reinforced composites having fibre reinforcing elements incorporated in a matrix. The invention is of particular relevance to the manufacture of shaped articles with fibre reinforcement within a resin matrix.
2. The Prior Art
As technology improves in relation to fibre reinforced composites, such materials are becoming used more and more in the manufacture of articles, especially for applications where high strength and light weight are desirable. In order to obtain the required strength characteristics, in the production of fibre reinforced composites the fibres must be controlled accurately as regards their positioning, orientation and amount. Sheet materials, i.e. essentially two dimensional products, are reasonably straightforward to manufacture, but three dimensional articles present considerable problems in regard to the control of the fibre reinforcement. For example, when a fibre mat or fabric is shaped by pressing in a mould, such as in a hemispherical mould cavity defined by male and female mould parts, there is a strong tendency for the fibre mat or fabric to fold and tear in an uncontrollable manner with the result that the fibre positioning will not be ideal for the article being produced and the article may have unacceptable weaknesses.
Reinforcement products used in the manufacture of fibre reinforced composites, such as woven fibre mats, frequently use continuous filaments in order to achieve maximum strength, but such filaments are incapable of stretching to conform to a three dimensional shape. Consequently, the amount to which such mats can deform is limited, especially if they are to be used to prepare a shaped reinforcement preform to be subsequently impregnated with and embedded in a matrix, such as thermosetting resin, metal or ceramic.
It is also known to use staple fibre yarns as reinforcement elements in reinforced composites, such as in spin winding processes wherein the yarn is wound onto a mandrel to build up layers of reinforcement. The yarns are generally produced with the fibres or filaments twisted so tightly that they are unable to slide relative to each other due to the friction between the fibres, and as a consequence the yarns will not elongate under tension and will break before stretching to any significant extent.
In WO 91/13195 (see U.S. Pat. No. 5,503,928) there is described a method of manufacturing a fibre reinforced composite wherein the reinforcement comprises a staple yarn of discontinuous fibres with low twist, which yarn is subjected to a preliminary controlled stretching with a view to obtaining desired characteristics in the finished composite. The stretched yarn can be used to produce a shaped preform which is subsequently impregnated with resin, e.g. during a separate moulding step. The need to produce a preform including a yarn which has been subjected to controlled stretching limits the potential of the described method for large scale production of shaped articles.